Age assignments and correlations of Miocene marine strata in California are usually made with reference to a sequence of benthic foraminiferal stages. During the past decade refined calcareous and siliceous planktonic microfossil biochronologies have been developed that provide a means for calibrating the California provincial stages to international standards while also testing the chronostratigraphic reliability of correlations derived by equating stage diagnostic benthic foraminifer assemblages in different areas. Diatoms are particularly useful for dating the upper, silica-rich part of the Monterey Formation whereas calcareous nannofossils are useful for dating the lower part of the Monterey Formation and older Miocene units.

Data from type sections show that the base of the Saucesian Stage is near the Oligocene-Miocene boundary of international usage, the base of the Relizian Stage falls at the base or in the lower part of the early Miocene Helicosphaera ampliaperta Zone (nannofossil), and the Luisian Stage essentially coincides with the middle Miocene Sphenolithus heteromorphus Zone (nannofossil) and the Denticulopsis lauta Zone to Subzone a of the Denticulopsis hustedtii--D. lauta Zone (diatom). The top of the type Mohnian Stage is within the latest Miocene to early Pliocene Nitzschia reinholdii Zone (diatom), whereas the type section of the Delmontian Stage is within Subzones b and c of the middle Miocene Denticulopsis hustedtii-D. lauta Zone. The type Delmontian is therefore equivalent to the lower Mo nian of most California sections.

Compilation of data from throughout California indicates some overlap in time of benthic foraminifer assemblages diagnostic of the Zemorrian and Saucesian Stages and of the Relizian and Luisian Stages. The transition from Luisian to Mohnian benthic foraminifer assemblages, however, consistently occurs near the top of the Sphenolithus heteromorphus Zone and within Subzone a of the Denticulopsis hustedtii-D. lauta Zone.

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